If you're still not able to create a group on your domain, the domain administrator has not enabled the Google Groups for Business feature (which is separate from the regular Google Groups product). Or, the administrator has not allowed users to create their own groups.

If users are unable to create their own groups, the domain administrator can still create the group from the Google Apps control panel (http://www.google.com/a/yourdomain.com). Click Groups -> Create a new group.